The present invention relates to an apparatus and to a method for operating a tape storage device, and to a related computer program product.
In tape storage systems, data typically is written to and read from a tape inserted into a tape storage device, which is also referred to as tape drive. The tape may be preformatted into data bands and servo bands extending next to each other in a motion direction of the tape. The servo bands comprise prerecorded servo patterns representing servo information which may be read by a servo reader of a head module for gaining information about a longitudinal position of the tape and a lateral position of the tape with respect to a reference position. Lateral in this context is meant orthogonal to a reference motion direction of the tape. A deviation of the lateral position from the reference position is also denoted as position error signal PES in this technical field. Such information, and specifically the PES, may be provided to a feedback controller, also denoted as lateral position feedback controller, which converts the PES into a control signal for a realigning between the tape head and the data tracks that are currently read or written for enabling a correct reading and writing of data. As a result, a closed-loop control system for a continuing alignment of the tape head and the tape is provided, which is also denoted as a closed-loop track-follow control system.
In state-of-the-art tape storage systems, additional information is encoded in the servo patterns of a servo band. For example, information as to the longitudinal position of the tape (which is also referred to as LPOS information) may be encoded. Such LPOS information may be encoded within a plurality of servo frames forming an LPOS word and may be obtained from the servo pattern without affecting the generation of the lateral position error signal and longitudinal velocity information.
In many tape storage systems, at least two servo bands are provided. At least two corresponding servo readers supply read-back signals when simultaneously reading the servo patterns from the at least two servo bands. In a preferred approach, the two servo bands at the top and the bottom of a data band are read simultaneously. Any additional information encoded into the servo bands may be derived from each of the servo bands. In one embodiment, the additional information may be derived by a dedicated servo channel. If the performance of the dedicated servo channel degrades, then the additional information may be derived from any of the other servo channels. However, in another embodiment, the additional information may be derived from a combination of the read-back signals of the two servo readers instead of switching the servo channel to overcome any performance degradation of the dedicated servo channel.